Substrate bonding method and semiconductor device

ABSTRACT

(a) A first Sn absorption layer ( 5 ) is formed on the principal surface of a first substrate ( 1 ), the first Sn absorption layer being made of metal absorbing Sn from AuSn alloy and lowering a relative proportion of Sn in the AuSn alloy. (b) A second Sn absorption layer ( 17 ) is formed on the principal surface of a second substrate ( 11 ) the second Sn absorption layer being made of metal absorbing Sn from AuSn alloy and lowering a relative proportion of Sn in the AuSn alloy. (c) A solder layer ( 7 ) made of AuSn alloy is formed at least on one Sn absorption layer of the first and second Sn absorption layers. (d) The first and second substrates are bonded together by melting the solder layer in a state that the first and second substrates are in contact with each other, with the principal surfaces of the first and second substrates facing each other.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and claims priority of Japanese PatentApplication No. 2007-162508 filed on Jun. 20, 2007, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION A) Field of the Invention

The present invention relates to a substrate bonding method of bondingtwo substrates together by using AuSn solder, and a semiconductor devicehaving a bonding region formed by AuSn solder.

FIG. 7A is a schematic cross sectional view showing a semiconductordevice before bonding, disclosed in JP-A-2006-86208. On a principalsurface of a support substrate 61, an Au layer 62, a Ti layer 63, a Nilayer 64 and an AuSn solder layer 65 are laminated in this order. On aprincipal surface of a temporary substrate 70, an emission layer 71, anAuZn layer 72, a TaN layer 73, an Al layer 74, a Ta layer 75 and an Aulayer 76 are laminated in this order. The Au layer 76 is in contact withthe AuSn solder layer 65, and heating is performed to melt, and then theAuSn solder layer 65 is solidified to bond the temporary substrate 70including the emission layer 71 to the support substrate 61. Afterbonding, the temporary substrate 70 is etched and removed. The AuZnlayer 72 has a function of reflecting light irradiated from the emissionlayer 71 and improving optical emission efficiency.

The Ni layer 64 prevents ball-up during re-solidification of theoverlying AuSn solder layer 65 after melting. The “ball-up” is aphenomenon that AuSn liquefied once at its eutectic temperature orhigher segregates on the support substrate 61, and partially rises. TheTaN film 73 prevents permeation of AuSn solder into the AuZn layer 72

FIG. 7B shows a bonding structure of a laser chip disclosed inJP-A-2006-332435. On a support substrate 61 made of silicon, a Ni layer64, an Au layer 66 and an AuSn solder layer 65 are laminated in thisorder. An Au layer 82 is formed on a bottom surface of a laser chip 81.The Au layer 82 is in contact with the AuSn solder layer 65, and heatingis performed to bond the laser chip 81 to the support substrate 61. TheNi layer 64 prevents ball-up during melting of the AuSn layer 65.

FIG. 7C shows a bonding stricture of a semiconductor device disclosed inJP-A-HEI-5-235323. On the bottom surface of a GaAs substrate 91, an AuGelayer 92, a Ni layer 93, an AuSn solder layer 94 and an Au layer 95 arelaminated in this order. The Au layer 95 is in contact with a packagesubstrate 61, and heating is performed to mount the GaAs substrate 91 onthe package substrate 61. The Ni layer 93 enhances adhesion of the AuSnsolder layer 94.

SUMMARY OF THE INVENTION

An AuSn solder layer is preferably made thin in order to lower thermalresistance of a bonding region and reduce material cost. However, as theAuSn layer is made thin, voids are likely to be generated on a bondedinterface. Voids generated on the bonded interface cause a loweredbonding strength and an increased thermal resistance. If a bondedinterface in a semiconductor device contains voids, thermal resistanceof the semiconductor device increases, a drive voltage rises, and thedevice lifetime characteristics are degraded.

An object of this invention is to provide a bonding method capable ofpreventing voids from generating on a bonded interface. Another objectof this invention is to provide a semiconductor device bonded by thisbonding method.

According to one aspect of the present invention, there is provided asubstrate bonding method comprising steps of:

(a) forming a first Sn absorption layer on a principal surface of afirst substrate, the first Sn absorption layer being made of metalabsorbing Sn from AuSn alloy and lowering a relative proportion of Sn inthe AuSn alloy;

(b) forming a second Sn absorption layer on a principal surface of asecond substrate, the second Sn absorption layer being made of metalabsorbing Sn from AuSn alloy and lowering a relative proportion of Sn inthe AuSn alloy;

(c) forming a solder layer made of AuSn alloy at least on one Snabsorption layer of the first and second Sn absorption layers; and

(d) melting the solder layer to bond the first and second substratetogether, in a state that the first and second substrates are in contactwith each other, with the principal surfaces of the first and secondsubstrates facing each other.

According to another aspect of the present invention, there is provideda semiconductor device comprising:

a first substrate;

an operation layer made of semiconductor and bonded to the firstsubstrate; and

a bonding layer made of alloy containing Au, Sn and another thirdelement, the bonding layer bonding the operation layer to the firstsubstrate,

wherein defining in the bonding layer a central region, a first regiondisposed between the central region and the first substrate, and asecond region disposed between the central region and the operationlayer, a relative proportions of elements in the bonding layer in such amanner that a relative proportion of Sn in the central region is smallerthan that in either of the first and second regions a relativeproportion of Au in the central region is larger than that in either ofthe first and second regions, and a relative proportion of the thirdelement in the central region is smaller than that in either of thefirst and second regions.

Since the first and second Sn absorption layers absorb Sn in the solderlayer, relative proportion of the Sn that the solder layer containslowers. A melting point of the solder layer after solidification risestherefore, and it is hard to be re-melted. Since the Sn absorptionlayers are disposed on both sides of the solder layer, Sn is absorbedfrom both sides of the solder layer. Absorption of Sn makes the meltedsolder layer gradually solidify so that voids do not remain on thebonded interface, and good bonding structure can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are cross sectional views of a substrate, illustrating asubstrate bonding method according to a first embodiment.

FIG. 2 is a table showing the structures and evaluation results ofsamples manufactured by the method of the first embodiment and a methodaccording to a modification of the first embodiment.

FIG. 3A is a cross sectional photographic view of a sample No. 3, andFIG. 3B is a graph showing measurement results of relative proportionsof elements in each region.

FIGS. 4A and 4B are cross sectional photographic views of a sample No. 6and a sample No. 10.

FIGS. 5A to 5C are cross sectional views of substrates to be bonded bybonding methods according to second to fourth embodiments.

FIGS. 6A to 6C are cross sectional views of substrates to be bonded bybonding methods according to fifth to seventh embodiments.

FIGS. 7A to 7C are cross sectional views of substrates to be bonded by aconventional soldering method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A substrate bonding method of the first embodiment will be describedwith reference to FIGS. 1A to 1F.

As shown in FIG. 1A, a Pt layer 3 is formed on the principal surface ofa first substrate 1 made of silicon (Si) and doped with n- or p-typedopant, and a Pt layer 2 is also formed on the bottom surface oppositeto the principal surface. In the first embodiment, a Si substrate wasused which had a B concentration of 3×10⁻¹⁸ cm⁻³ or higher (specificresistance of 0.02 Ωcm or lower) and had a (100) crystal surface.

The Pt layer is formed, for example, resistance heating evaporation,electron beam evaporation, sputtering or the like. A thickness of eachof the Pt layers 2 and 3 is preferably 25 nm or thicker in order to usethe Pt layer as an ohmic electrode. Since a work function of Pt islarger than that of p-type Si, an ohmic contact can be retained betweenthe Pt layers 2, 3 and the first substrate 1. Since a heating process isexecuted at a later thermocompression bonding process, silicidationprogresses so that an ohmic contact and low contact resistance can bemaintained. The first substrate 1 may be made of conductive materialhaving a high thermal conductivity such as Cu, instead of Si.

On the Pt layer 3, a Ti layer 4 of 150 nm thick is formed. On the Tilayer 4, a first Sn absorption layer 5 of 100 nm thick made of Ni isformed. The Ti layer 4 and first Sn absorption layer 5 are formed, forexample, by electron beam evaporation or sputtering. The Ti layer 4enhances adhesion of the overlying first Sn absorption layer 5.

On the first Sn layer 5, an Au layer 6 is formed, and on this Au layer6, a solder layer 7 made of AuSn alloy is formed. The Au layer 6 andsolder layer 7 are formed, for example, by resistance heatingevaporation or sputtering. Thicknesses of the Au layer 6 and solderlayer 7 are, e.g., 30 nm and 600 nm, respectively. A relative proportionof Au and Sn contained in the solder layer 7 is about 8:2 by weight andabout 7:3 by number of atoms. Additive other than Au and Sn may be addedto the solder layer 7.

As shown in FIG. 1B, an operation layer 12 including a plurality ofsemiconductor layers is epitaxially grown on the principal surface of asecond substrate 11 made of semiconductor. The operation layer 12 is anemission layer which emits light having a wavelength intrinsic tosemiconductor material by injecting electrons and holes for example. Asthe material of the second substrate 11, semiconductor material isselected having a crystal structure and a lattice constant allowing thesemiconductor material of the operation layer 12 to be epitaxially grownat a high quality.

For example, if the operation layer 12 is made to have a multiplequantum well structure having well layers and barrier layers made ofAlGaInP-based compound semiconductor, a GaAs substrate is used as thesecond substrate 11. The operation layer 12 may have a homo pn junctionstructure, a double hetero structure or a single hetero structure. Thesemiconductor emission layer may be sandwiched between an n-type cladand a p-type clad.

On the operation layer 12, a reflection electrode layer 13 is formed.The reflection electrode layer 13 has a function of improving lightemission efficiency by reflecting light generated in the operation layer12, in addition to a function of an electrode. The reflection electrodelayer 13 is made of metal capable of ohmic contact with the operationlayer 12. If a surface layer of the operation layer 12 on the side ofthe reflection electrode layer 13 is made of p-type AlGaInP, AuZn can beused as the material of the reflection electrode layer 13. In this case,the reflection electrode layer 13 can be formed by resistance heatingevaporation, electron beam evaporation or sputtering. A thickness of thereflection electrode layer 13 is, for example, 300 nm.

On the reflection electrode layer 13, a TaN layer 14 of 100 nm thick isformed by reactive sputtering. After the TaN layer 14 is formed, heattreatment is performed at about 500° C. in a nitrogen atmosphere. Thisheat treatment makes an alloy of the reflection electrode layer 13 ofAuZn and the surface layer of p-type AlGaInP of the operation layer 12.In the result, good ohmic contact can be obtained. The TaN layer 14prevents infiltration of AuSn eutectic melted in a later process intothe reflection electrode layer 13.

On the TaN layer 14, a TiW layer 15 of 100 nm thick and a TaN layer 16of 100 nm thick are formed by reactive sputtering. On the TaN layer 16,a second Sn absorption layer 17 of Ni is formed by electron beamevaporation or sputtering. A thickness of the second Sn absorption layer17 is set to 300 nm. On the second Sn absorption layer 17, an Au layer18 of 30 nm thick is formed by resistance heating evaporation orsputtering. The Au layer 18 prevents oxidation of the Ni layer 17.

As shown in FIG. 1C, the first substrate 1 and second substrate 11 aredisposed with their principal surfaces being faced each other. As shownin FIG. 1D, the AuSn solder layer 7 on the first substrate 1 and the Aulayer 18 on the second substrate 11 are in contact with each other, andthermocompression bonding is performed in a nitrogen atmospheres. Thethermocompression bonding conditions are as follows:

Pressure: about 1 MPa

Temperature: 320 to 370° C.

Thermocompression bonding time: 10 minutes

As shown in FIG. 1E, the AuSn solder layer 7 melts, Au in the Au layers6 and 18 and Ni in the first and second Sn absorption layers 5 and 17are dissolved in the melted solder layer 7, and Au and Sn in the Aulayers 6 and 18 and solder layer 7 are diffused into and absorbed in thefirst and second Sn absorption layers 5 and 17. As the melted solderlayer 7 is solidified, a bonding layer 20 made of AuSnNi is thereforeformed. After bonding, the second substrate 11 is removed. If the secondsubstrate 11 is made of GaAs, the second substrate 11 can be removed, bywet etching using mixture liquid of aqueous solutions of ammonia andhydrogen peroxide. Instead of wet etching, the second substrate may beremoved by dry etching, chemical mechanical polishing, mechanicalpolishing or the like.

As shown in FIG. 1F, the surface of the operation layer 12 is exposed.Front side electrodes 30 are formed in partial areas of the exposedsurface. If n-type AlGaInP is exposed on the operation layer 12contacting the front side electrodes 30, AuSnNi, AuGeNi, AuSn or AuGemay be used as the material of the front side electrodes 30. The frontside electrodes 30 can be formed by film formation by resistance heatingevaporation, electron beam evaporation, sputtering or the like, andlift-off. After the front side electrodes 30 are formed, heat treatmentis performed at about 400° C. in a nitrogen atmosphere to ensure ohmiccontact.

Carriers are supplied to the operation layer 12 from the Pt layer 2 onthe bottom surface of the first substrate 1 and front side electrodes30. Light generated in the operation layer 12 is emitted through thesurface on which the front side electrodes 30 are formed.

A sample was manufactured by the substrate bonding method of the firstembodiment, and a plurality of samples were manufactured by changingthicknesses of the solder layer 7, Au layers 6 and 18 and second Snabsorption layer 17. A thickness of the first Sn absorption layer 5 wasset to 100 nm. Bonding structures of these samples were evaluated.

FIG. 2 shows structures and evaluation results of the manufacturedsamples. T(Au) shown in FIG. 2 indicates a total thickness of the Aulayers disposed between the first and second Sn absorption layers 5 and17, T(AuSn) indicates a thickness of the solder layer 7, and T(Ni)indicates a total thickness of the first and second Sn absorption layers5 and 17. The sample manufactured by the first embodiment methodcorresponds to a sample No. 1 in FIG. 2.

FIG. 3A is a cross sectional photographic view of the bonding layer of asample No. 3. It can be seen that the bonding layer made of AuSnNi ispartitioned into four regions being different in appearance in athickness direction.

FIG. 3B shows measurement results, by Auger electron spectroscopy ofrelative proportions of elements in the four regions A to D at differentpositions in the thickness direction of the bonding layer. An abscissarepresents relative proportions in number of atoms in the unit of “%”.Regions A to D are disposed in the bonding layer 20 sequentially in theorder from the first substrate 1 side.

The relative proportion of Sn in the region B positioned roughly in thecenter of the bonding layer 20 is smaller than that in either of theregion A or the region C. The relative proportion of Au in the centralregion B is larger than that in either of the region A or the region C.The relative proportion of Ni in the central region B is smaller thanthat in either of the region A or the region C. In the regions A and C,the relative proportion of Ni is larger than either of the relativeproportion of Au or the relative proportion of Sn. In the central regionB, the relative proportion of Au is larger than the relative proportionof Ni, and the relative proportion of Ni is larger then the relativeproportion of Sn. It has been confirmed that the region D becomes thinas the second Sn absorption layer 17 is made thinner than that of thesample 3.

It can be understood that Sn in the solder layer 7 is absorbed in thefirst and second Sn absorption layers 5 and 17, because the relativeproportion of Sn in the region B is smaller than those in the regions Aand C on both sides of the region B. The relative proportion of Au inthe solder layer 7 before melting was 70% in number of atoms. Incontrast, the relative proportion of Au in the solder layer 7 afterre-solidification is 90% or higher in number of atoms. It can beunderstood from this result that the first and second absorption layers5 and 17 preferentially-absorb Sn compared to Au. The reason why therelative proportion of Sn in the region D is smaller than that in theregion C may be ascribed to that Sn is not sufficiently diffused to theopposite surface of the second Sn absorption layer 17 because the secondSn absorption layer 17 of 300 nm thick is thicker than the first Snabsorption layer 5 of 100 nm thick.

Because the relative proportion of Au in the region B is higher thanthat in the solder layer 7 before melting, a melting point of the regionB is higher than a melting point of the original solder layer 7. Morespecifically, a melting point of AuSn alloy containing Au, the relativeproportion of which is 70%, is about 280° C., whereas a melting point ofAuSn alloy containing Au, the relative proportion of which is 90%, isabout 900° C. Therefore, when a semiconductor device after bonding ismounted on a package substrate or the like, the bonding layer is hard tobe re-melted. The property of difficulty in re-melting after bonding iscalled “re-melting durability”. In the embodiment described above,re-melting durability can be enhanced by increasing the relativeproportion of Au in the solder layer after solidification.

FIGS. 4A and 4B are cross sectional photographic views of samples Nos. 6and 10. Voids 50 were observed on the bonded interface of either sample.However, the sample No. 6 had few voids, and sufficient tight adhesionwas retained. The sample No. 10 had many voids, and sufficient tightadhesion was not retained.

In FIG. 2, evaluation of the sample in which voids were not observed atthe bonding interface is represented by “O”, evaluation of the sample inwhich although voids were observed, sufficient tight adhesion was ableto be retained is represented by “Δ”, and evaluation of the sample inwhich sufficient tight adhesion was not able to be retained because ofmany voids is represented by “x”. It is possible to prevent generationof voids at the bonding interface by setting T(Au)/T(AuSn) to 0.22 orsmaller. Sufficient tight adhesion can be retained by settingT(Au)/T(AuSn) to 0.39 or smaller.

Description will now be made on the cause-and-effect relationshipbetween a ratio of a total thickness of the Au layers to a thickness ofthe solder layer 7 and generation of voids.

A melting point of AuSn solder (Au:Sn=7:3 in number of atoms) of thesolder layer 7 is about 280° C., and rises even if the relativeproportions of Au and Sn shift in either direction. If the relativeproportions shifts in a direction of increasing the relative proportionof Au, a rise tendency of the melting point is steep. As the Au layerbecomes relatively thick, Au atoms in the Au layer dissolve in themelted AuSn solder layer when the AuSn solder layer 7 melts, and therelative proportion of Au in the dissolved region becomes high. Asolidification speed therefore increases, and the solder layer issolidified before air bubbles generated on the bonded interface arereleased from the bonded interface. In the result, voids are consideredto be generated on the bonded interface.

If the Au layer is thin, it is not dominant that the relative proportionof Au rises because Au of the Au layer dissolves in the melted solderlayer 7, but it is dominant that the relative proportion of Au risesbecause Sn is absorbed in the first and second Sn absorption layers 5and 17. Since Sn is gently absorbed in the first and second Snabsorption layers 5 and 17, a rise in the relative proportion of Au inthe melted solder layer is also gentle. Therefore, a time until thesolder layer 7 is solidified is prolonged. It can be considered thatgeneration of voids is prevented because it is possible to retain thetime required for air bubbles generated on the bonded interface to moveto the ends of the bonded interface and be released to the external.

Next, description will be made on material other than Ni to be used asthe material of the first and second Sn absorption layers 5 and 17.

The first and second Sn absorption layers 5 are required to have theproperty of preferentially absorbing Sn from melted AuSn solder andmaking the relative proportion of Sn in AuSn solder after solidificationbe smaller than that of AuSn solder before melting. With this property,a melting point of AuSn solder rises and re-melting durability can beenhanced. In order to avoid ball-up during melting of AuSn solder, it isrequired that AuSn solder wettability is high. Such material includes Ptand Pd, other than Ni.

Next, description will be made of a preferable thickness of the firstand second Sn absorption layers 5 and 17 made of Ni.

If the first and second Sn absorption layers 5 and 17 are too thin ascompared to the solder layer 7, Sn in the solder layer 7 cannot beabsorbed sufficiently. In this case, the relative proportions of Au andSn in the solder layer 7 after solidification are hardly changed fromthose before melting in some regions. As the regions in which therelative proportions are not changed are left, sufficient re-meltingdurability cannot be retained. Of the samples 1 to 5, 12 and 13evaluated as “O”, the sample No. 13 has a minimum T(Ni)/T(AuSn) value of0.41. This sample No. 13 had a reduced relative proportion of Sn in thesolder layer after solidification, and retained sufficient re-meltingdurability. It can therefore be considered that sufficient re-meltingdurability can be retained by setting T(Ni)/T(AuSn) to 0.41 or larger.In the sample No. 11 having T(Ni)/T(AuSn) of 0.33 adhesion on the bondedinterface was weak, and partial stripping occurred.

The first Sn absorption layer 5 has preferably a thickness of 100 nm orthicker in order to improve wettability with respect to the solder layer7 and suppress ball-up. It has been confirmed that the samples Nos. 1 to5 having the first Sn absorption layer 5 of 100 nm thick can preventball-up.

A thickness of the second Sn absorption layer 17 is required to bedetermined from the viewpoint of not generating voids on the bondedinterface, because the surface thereof becomes the bonded interface. Forexample, in order to retain sufficient re-melting durability,T(Ni)/T(AuSn) is set to 0.41 or larger. T(Ni) is a total thickness ofthe first and second Sn absorption layers 5 and 17. Therefore, as thefirst Sn absorption layer 5 is made thick, the second Sn absorptionlayer 17 can be thinned relatively.

At the initial stage of bonding, a portion of the second Sn absorptionlayer 17 melts together with the solder layer 7 and takes a state thatair bubbles are contained. As described earlier, given a sufficient timeof a melting state, air bubbles move to the ends of the bonded interfaceand are released to the external. However, if the second Sn absorptionlayer 17 is made too thin, the whole region of the second Sn absorptionlayer 17 melts together with the solder layer 7, and the TaN layer 16being in contact with the second Sn absorption layer 17 becomes incontact with the air bubbles. Since the TaN layer 16 has poorwettability with respect to solder, a state where the TaN layer is incontact with the air bubbles is rather stable. Air bubbles are thereforedifficult to move, the solder layer is solidified holding the airbubbles therein, and voids are generated on the bonded interface.

As described above, when the layer being in contact with the second Snabsorption layer 17 is made of material having poor wettability withrespect to solder, it is preferable to thicken the second Sn absorptionlayer 17 to such a degree that a region not melted remains in a partialregion of the second Sn absorption layer 17 on the side opposite to thesolder layer 7. For example, when a thickness of the second Snabsorption layer 17 is 150 nm or thicker, it is possible to prevent airbubbles from becoming resident. When the layer being in contact with thesecond Sn absorption layer 17 is made of material having goodwettability with respect to melted solder, the second Sn absorptionlayer 17 may be made thinner than 150 nm.

Considering various circumstances, a thickness of the second Snabsorption layer 17 is preferably set to 150 nm or thicker in order toprevent generation of voids on the bonded interface. The sample No. 13having the second Sn absorption layer 17 of 150 nm thick has a goodbonded interface without voids.

When the solder layer 7 of AuSn is made thick, a region havingapproximately the same relative proportions of Au and Sn as those in thesolder layer 7 appears near the central area of the region B shown inFIG. 3B in the thickness direction. Also in this case, the bonding layercontains the regions A and C having Ni as their main compositions andthe region B between the regions A and C having Au as its maincomposition, so that a good bonded interface without voids can beobtained.

FIGS. 5A to 5C are cross sectional views of substrates before bonding tobe used by substrate bonding methods according to the second to fourthembodiments. Namely, FIGS. 5A to 5C correspond to FIG. 1C of the firstembodiment.

As show in FIG. 5A, in the second embodiment, the surface of an AuSnsolder layer 7 is covered with an Au layer 8. In this case, a totalthickness T(Au) of Au layers disposed between a first Sn absorptionlayer 5 and a second Sn absorption layer 17 is equal to a totalthickness of Au layers 6, 8 and 18.

As shown in FIG. 5B, in the third embodiment, an AuSn solder layer isformed not on the first substrate 1 but on the second substrate 11. Onthe first substrate 1, an Au layer 6 formed on a first Sn absorptionlayer 5 is exposed. An AuSn solder layer 7 a is formed on the surface ofan Au layer 18 on the second substrate 11, and the surface of the AuSnsolder layer 7 a is covered with an Au layer 8 a.

As shown in FIG. 5C, in the fourth embodiment, an AuSn solder layer isformed on either of a first substrate 1 or a second substrate 11. On thefirst substrate 1, a solder layer 7 b is formed on an Au layer 6, andthe surface of the solder layer 7 b is covered with an Au layer 8 b. Onthe second substrate 11, a solder layer 7 c is formed on the surface ofan Au layer 18, and the surface of the solder layer 7 c is covered withan Au layer 8 c. In this case, a total thickness T(AuSn) of AuSn solderlayers is equal to a total of a thickness of the solder layer 7 b on thefirst substrate 1 and a thickness of the solder layer 7 c on the secondsubstrate 11.

FIGS. 6A to 6C are cross sectional views of substrates to be used bysubstrate bonding methods according to the fifth to seventh embodiments.Namely, the cross sectional views correspond to the cross sectional viewshown in FIG. 1C of the first embodiment.

As shown in FIG. 6A, in the fifth embodiment, the Au layer 6 and 18shown in FIG. 1C of the first embodiment are not disposed. The sampleNo. 12 shown in FIG. 2 corresponds to the structure of the fifthembodiment, and its T(Au)/T(AuSn) is 0. As shown in FIG. 6B, in thesixth embodiment, the Au layers 6, 8 a and 18 shown in FIG. 5B of thethird embodiment are not disposed. As shown in FIG. 6C, in the seventhembodiment, the Au layers 6, 8 b, 8 c and 18 shown in FIG. 5C of thefourth embodiment are not disposed,

As described earlier, even the structures of the second to seventhembodiments are expected to present same advantages to those of thefirst embodiment.

In the third embodiment shown in FIG. 5B and in the sixth embodimentshown in FIG. 6B, the surface of the first Sn absorption layer 5 on thefirst substrate 1 or the surface of the overlying Au layer 6 constitutesthe bonded interface. Therefore, in order to prevent generation of voidson the bonded interface, it is preferable that a thickness of the firstSn absorption layer 5 is set to 150 nm or thicker. It is sufficient if athickness of the second Sn absorption layer 17 is 100 nm or thicker.

In the fourth embodiment shown in FIG. 5C and in the seventh embodimentshown in FIG. 6C, either of the first Sn absorption layer 5 or thesecond Sn absorption layer 17 is spaced apart from the bonding interfaceso that it is sufficient if a thickness of each layer is 100 nm orthicker. However, also in the second to seventh embodiments, in order toretain sufficient re-melting durability, T(Ni)/T(AuSn) is preferably setto 0.41 or larger, as in the case of the first embodiment.

In each sample shown in FIG. 2, although a thickness of the AuSn solderlayer 7 is set to 600 nm to 950 nm, different thicknesses may be used.Conventionally, a thickness of the AuSn solder layer has been setthicker than 1 μm in order not to generate voids and in order to retainsufficiently strong bonding. By adopting the above-described embodimentmethods, good bonding without generation of voids can be obtained evenif a thickness of the AuSn solder layer 7 is set to 1 μm or thinner.

In each sample shown in FIG. 2, by thinning the AuSn solder layer 7,damages to a blade of a dicing apparatus were able to be reduced in adicing process of semiconductor devices, although damages occurred whena conventional thick solder layer was used.

Further, in each sample shown in FIG. 2, as the AuSn solder layer 7 isthinned, the relative proportions of Au and Sn in the bonding layerchanges greatly after bonding, as compared to using a conventional thicksolder layer. Re-melting durability of the bonding layer can thereforebe improved. When a semiconductor device is mounted on a circuit boardor the like by reflow soldering, various problems can be mitigatedincluding re-melting by heat during soldering, position displacement,stripping, sticking out of solder from an edge and the like, to becaused by re-melting.

In the embodiments described above, although the relative proportions ofAu and Sn in the AuSn solder layer is set to about 7:3 in number ofatoms, AuSn alloy having other relative proportions may also be used. Itis preferable to set the relative proportions satisfying the conditionthat a melting point of the solder layer 7 having a lowered relativeproportion of Sn after bonding is higher than a melting point of thesolder layer 7 before melting. With such a relative proportions,re-melting durability of the bonding layer can be enhanced.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It will be apparent to those skilled in the art that othervarious modifications, improvements, combinations, and the like can bemade.

1. A substrate bonding method comprising steps of: (a) forming a firstSn absorption layer on a principal surface of a first substrate, thefirst Sn absorption layer being made of metal that absorbs Sn from AuSnalloy and lowers a relative proportion of Sn in the AuSn alloy; (b)forming a second Sn absorption layer on a principal surface of a secondsubstrate, the second Sn absorption layer being made of a metal thatabsorbs Sn from AuSn alloy and lowering a relative proportion of Sn inthe AuSn alloy; (c) forming a solder layer made of AuSn alloy at leaston one of the first and second Sn absorption layers; and (d) melting thesolder layer to bond the first and second substrates together, in astate in which the principal surfaces of the first and second substratesface each other; wherein in step (d), Sn atoms in the solder layerdiffuse into the first Sn absorption layer and the second Sn absorptionlaver, and a metal element of the first Sn absorption layer and secondSn absorption layer is dissolved in the melted solder layer, therebyforming a bonding layer including the first Sn absorption layer, thesecond Sn absorption layer, and the solder layer, with the metal elementdistributed across an entire thickness of the bonding layer.
 2. Thesubstrate bonding method according to claim 1, wherein the first andsecond Sn absorption layers are made of Ni.
 3. The substrate bondingmethod according to claim 2, wherein the solder layer is formed only onone of the first and second Sn absorption layers, and a thickness of theSn absorption layer on which the solder layer is formed is at least 100nm.
 4. The substrate bonding method according to claim 3, wherein athickness of the Sn absorption layer on which the solder layer is notformed is at least 150 nm.
 5. The substrate bonding method according toclaim 2, wherein a total thickness of the first Sn absorption layer andthe second Sn absorption layer is equal to or greater than 0.41 times athickness of the solder layer.
 6. The substrate bonding method accordingto claim 2, further comprising, after step (a), covering a surface ofthe first Sn absorption layer with an Au layer.
 7. The substrate bondingmethod according to claim 2, further comprising, after step (b),covering a surface of the second Sn absorption layer with an Au layer.8. The substrate bonding method according to claim 2, furthercomprising, after step (c), covering a surface of the solder layer withan Au layer.
 9. The substrate bonding method according to claim 2,further comprising: after step (a), covering a surface of the first Snabsorption layer with a first Au layer; after step (b), covering asurface of the second Sn absorption layer with a second Au layer;wherein a total thickness of the Au layers disposed between the firstand second Sn absorption layers is equal to or less than 0.39 times atotal thickness of the solder layer formed on at least one of the firstand second Sn absorption lavers, in the state in which the principalsurfaces of the first and second substrates face each other in step (d).10. The substrate bonding method according to claim 1, wherein, in step(d), the Sn atoms in the solder layer diffuse into the first and secondSn absorption layers so that a relative proportion of Sn in the solderlayer after solidification is smaller than a relative proportion of Snin the solder layer before the melting.
 11. The substrate bonding methodaccording to claim 1, wherein a thickness of the solder layer before themelting is 1 μm or less.
 12. The substrate bonding method according toclaim 1, wherein the first substrate comprises an operation layer, andthe substrate bonding method further comprises: forming an electrode onthe operation layer after step (d); and performing heat treatment toensure ohmic contact between the operation layer and the electrode. 13.The substrate bonding method according to claim 12, wherein theoperation layer comprises AlGaInP-based compound semiconductor.
 14. Thesubstrate bonding method according to claim 9, further comprising, afterstep (c), covering a surface of the solder layer with a third Au layer.